Maintaining appropriate analyte levels in the bloodstream of mammals, including humans, is extremely important, and failure to do so can lead to serious health problems and even death. For example, in diabetic patients, malfunction of the pancreas can lead to uncontrolled blood glucose levels, possibly resulting in hypoglycemic or hyperglycemic shock. To compensate for this and to maintain an appropriate blood glucose level, diabetics must receive timely and correct doses of insulin. Similarly, many other analytes commonly are measured in the blood of humans and in other fluids, for the purpose of determining an appropriate response to any measured surplus or deficiency of the analyte.
One method of measuring an analyte concentration in the blood of a mammal is to use an implantable sensor to measure the concentration, and a number of previous patented inventions relate to various aspects of such sensors. U.S. Pat. No. 5,711,861 to Ward et al. claims a disc-shaped sensor device having multiple anode/cathode pairs of electrodes for taking redundant analyte measurements. U.S. Pat. No. 6,212,416 to Ward et al. adds a coating to the sensor to inhibit formation of collagen or to enhance the sensitivity of the sensor in the presence of the analyte, and claims multiple redundant sensors (as opposed to a single sensor with multiple electrode pairs). U.S. Pat. No. 6,466,810 to Ward et al. claims a sensor with a single cathode and a plurality of anodes on each side, to provide redundant measurements without requiring multiple cathodes.
Once analyte measurements have been obtained with a sensor—whether the sensor is implantable or otherwise—a response often must be determined, typically in the form of a fluid infusion rate to alter the analyte concentration to a more desirable level. The infused fluid may contain the analyte itself, or it may contain a substance the presence of which affects the analyte level. For example, if the measured analyte is glucose, the infused fluid may contain glucose, or it may contain insulin.
Typically, an algorithm is used to determine a fluid infusion rate from analyte measurements, and several such algorithms are known. For example, a glucose-controlled insulin infusion system incorporating a proportional derivative (PD) method is disclosed in U.S. Pat. No. 4,151,845 to Clemens. U.S. Pat. No. 6,558,351 to Steil et al. claims an insulin infusion system using a proportional integral derivative (PID) algorithm that takes a patient's history of glucose levels into account when determining the infusion rate, by integrating the difference between the measured glucose level and the desired glucose level from some prior time up to the present. U.S. Pat. No. 6,740,072 to Starkweather et al. adjusts the parameters of the insulin infusion algorithm dynamically in response to exercise, sleep, and other external events.
However, despite the use of various algorithms to determine a response to a measured analyte concentration, no algorithm has been developed that takes into account both current and prior analyte levels in a manner that adequately reflects the dynamic nature of the measured concentration. In the case of glucose measurements and insulin infusion, for example, none of the previously developed algorithms are able to simulate completely the normal insulin response of a healthy pancreas. Thus, a need exists for an improved system for measuring an analyte concentration, processing the measurements using an algorithm that adequately takes into account the dynamics of the analyte, and determining a response.